Triazacyclononane-based phosphinate ligand and its use for molecular imaging

ABSTRACT

The present invention relates to the field of molecular imaging, i.e. nuclear and fluorescent imaging using metal ion radionuclides in combination with chelates highly functionalized with peptidic, nonpeptidic or protein ligands or additional signalling moieties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT/EP2012/050123 filed on Jan. 5, 2012, which claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/460,954 filed on Jan. 10, 2011 and under 35 U.S.C. 119(a) to patent application Ser. No. 11/150,496.5 filed in Europe on Jan. 10, 2011, all of which are hereby expressly incorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to the field of molecular imaging, i.e. nuclear and fluorescent imaging using metal ion radionuclides in combination with chelates highly functionalized with peptidic, nonpeptidic or protein ligands or additional signalling moieties.

BACKGROUND ART

Metal radionuclides are frequently used in nuclear imaging and therapy. Fluorescently labelled ligands are commonly used for optical imaging in animal models or in vivo imaging during i.e. surgical interventions. Related tracers are usually formed from a metal binding group (chelate ligand) or fluorescent units and moieties, which are bound, with or without linkers, to one or more targeting vectors.

Currently, DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) is the most frequently used compound for the purpose of nuclear imaging with radiometals. It forms stable complexes with many transition metal ions as well as lanthanide ions. Very frequently, one of the acetic acid side arms of this molecule is used to for conjugation of DOTA to the targeting vector resulting in the formation of an amide. For imaging purposes, the metal ion is finally added in the last step, thereby forming the complex which serves as the tracer or radiopharmaceutical. Particularly for complexation of lanthanide ions, the most established chelators are DOTA and derivatives of this structure, such as 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A). For smaller metal ions such as Ga(III), bifunctional chelators based on the 1,4,7-triaza-1,4,7-triacetic acid (NOTA) structural motif, such as NODAGA (1,4,7-triaza-1-glutaric acid-4,7-diacetic acid), have recently gained popularity because the resulting complexes are more thermodynamically stable and are formed more easily.

All of these chelators bind the radioactive metal ion by coordination on the nitrogen atoms of the azamacrocycle backbone and on the deprotonated carboxyl groups of the acetic acid substituents. These carboxylic acid moieties thus have to be deprotonated in order to act as coordination sites, which is why a pH value exceeding their pK_(a) of approx. 3.5-4.5 must be maintained during the labelling procedure. Labelling at a lower pH is substantially hampered. In case of the radionuclide ⁶⁸Ga(III) this is somewhat contradictory due to the fact that at pH>3, formation of colloidal Ga(OH)₃ commences which is also inhibiting the complex formation. For labelling of all iminopolyacetic acid ligands, like the above mentioned ones, a careful adjustment of pH value to 3-3.5 is thus mandatory. In addition, labelling of DOTA-like structures requires either heating, usually up to 80-95° C., or comparably high ligand concentrations, in the range of for example 1 mM.

In order to prepare bioconjugates of chelators, that is, molecules consisting of a targeting vector covalently bound to a chelating unit, the use of protecting groups on either side is mandatory in most cases according to the prior art. Particularly in case of DOTA or DO3A, the carboxylate moieties intended for metal complexation are often protected during amide coupling, see for example Schottelius M, Schwaiger M, Wester H J. Rapid and high-yield solution phase synthesis of DOTA-Tyr³-octreotide and DOTA-Tyr³-octreotate using unprotected DOTA. Tetrahedron Lett. 2003, 44 (11), 2393-2396. Tris-tert-butyl esters of these compounds are thus employed for conjugation. In most cases, an additional subsequent deprotection step is necessary in order to obtain the desired fully deprotected bioconjugate.

In the context of bioconjugates, the term multimer refers to molecules which comprise more than one targeting vector of the same kind. Multimers are desirable because in comparison to monomers, they can exhibit increased affinity to the respective target, thus often resulting in higher target uptake, higher target/background ratios, and thus better images. For preparation of multimeric radiometal tracers, DOTA-like chelators are usually bound to a linker which allows more than one targeting vector to be bound. The assembly of such molecules usually involves multistep syntheses with low overall yield. In addition and especially in the case of Ga-68 labelled peptides, two carboxylic groups of DOTA have been used for the formation of dimers by amide formation.

Combination of more than one imaging modality is usually referred to as multimodal imaging. Imaging technologies that can be combined include positron emission tomography (PET), single photon emission computed tomography (SPECT), planar scintigraphy, fluorescence imaging, magnet resonance tomography (MRT), optical imaging (either fluorescence or bioluminescence imaging) and combinations thereof, i.e. PET/CT and PET/MRT. To perform multimodal imaging, tracers are required possessing more than one reporter unit, e.g. combining the presence of a chelate unit for radiometals (for e.g. PET) with a so called fluorophor (for fluorescence). For such an approach, the presence of targeting vectors in the same molecule is, however not mandatory, still necessary for functional imaging.

Recently, a novel chelating unit based on 1,4,7-triazacyclononane, PrP9 (I), has been introduced, see for example Notni J., Hermann P., Havlickova J., Kotek J., Kubicek V., Plutnar J., Loktionova N., Riss P. J., Rosch F., Lukes I., A Triazacyclononane-Based Bifunctional Phosphinate Ligand for the Preparation of Multimeric Ga-68 Tracers for Positron Emission Tomography. Chemistry—A European Journal. 2010; 16(24):7174-7185.

3PrP9 (I) is particularly suitable for complexation of smaller radiometal ions, such as ⁶⁸Ga³⁺. In contrast to the above mentioned chelators bearing carboxylates, phosphinic acid moieties exhibit pKa values below 1. Thus, labelling can be performed at pH<2, thus circumventing formation of colloids.

AIMS OF THE INVENTION

The invention aims at chelate ligands for radiometals, which are equipped with biomolecules, such as cyclic peptides, to be used for molecular imaging and/or therapy.

In addition, the invention aims at said chelate ligands, bearing additional reporter units for other imaging modalities such as fluorescence imaging.

The invention further aims at the fast and high yield radiolabelling of the aforementioned chelators and their derivatives with radiometals, preferably with ⁶⁸Ga, particularly preferably with ⁶⁸Ga³⁺, at very low chelator concentrations in the reaction mixture, resulting in radiopharmaceuticals with exceptionally high specific activity.

Another aim of the invention is the application of said radiopharmaceuticals for nuclear and fluorescence imaging purposes.

DISCLOSURE OF THE INVENTION

The inventors of the present invention have found that chelate ligands which are based on amide-functionalized 1,4,7-triazacyclononane-1,4,7-tris[methyl(2-carboxyethyl)phosphinic acid](II)

give rise to corresponding nuclear chelates which may advantageously used for molecular imaging and/or therapy, bearing additional reporter units for other imaging modalities such as fluorescence imaging. In addition the chelate ligands according to the present invention make fast and high yield radiolabelling of the aforementioned chelators and their derivatives with radiometals, preferably with ⁶⁸Ga, particularly preferably with ⁶⁸Ga³⁺, at very low chelator concentrations in the reaction mixture, possible. These circumstances result in radiopharmaceuticals with exceptionally high specific activity.

The present invention therefore relates to chelate ligands according to general formula (II)

wherein

-   Z¹ is OH or NR¹R⁴, -   Z² is OH or NR²R⁵, -   Z³ is OH or NR³R⁶, -   R¹, R², R³ is independently of another selected from the group     consisting of linear or cyclic, substituted or unsubstituted,     aliphatic, heteroaliphatic, aromatic, heteroaromatic, saturated or     unsaturated radicals, wherein said R¹, R² and/or R³ is optionally     attached to the core molecule via at least one linking group, -   R⁴, R⁵, R⁶ is independently of another selected from the group     consisting of hydrogen, linear or cyclic, substituted or     unsubstituted, aliphatic, heteroaliphatic, aromatic, heteroaromatic,     saturated or unsaturated radicals,     wherein at least one of Z¹, Z² and Z³ is different from OH.

In a preferred embodiment Z¹, Z² and Z³ are different from OH, meaning that Z¹ is NR¹R⁴, Z² is NR²R⁵ and Z³ is NR³R⁶.

In another embodiment of the present invention two of Z¹, Z² and Z³ are different from OH, meaning that Z¹ is NR¹R⁴ and Z² is NR²R⁵ and Z³ is OH, or Z¹ is NR¹R⁴ and Z² is OH and Z³ is NR³R⁶, or Z¹ is OH and Z² is NR²R⁵ and Z³ is NR³R⁶.

In another embodiment of the present invention one of Z¹, Z² and Z³ is different from OH, meaning that Z¹ is NR¹R⁴ and Z² is OH and Z³ is OH, or Z¹ is OH and Z² is NR²R⁵ and Z³ is OH, or Z¹ is OH and Z² is OH and Z³ is NR³R⁶.

According to the present invention, the core molecule is PrP9 according to formula (I). Compounds according to general formula (II) are in general obtained by reaction of compound of general formula (I) and amines R¹R⁴NH, R²R⁵NH, R³R⁶NH, wherein R¹, R², R³, R⁴, R⁵, R⁶ have the same meanings as mentioned above.

The present invention further relates to a process for the preparation of chelate ligands according to the present invention by reaction of chelate ligands of formula (I)

with amines of general formula R¹R⁴NH, R²R⁵NH, R³R⁶NH, wherein R¹, R², R³, R⁴, R⁵, R⁶ have the same meanings as mentioned above.

The present invention further relates to a chelate complex which comprises at least one chelate ligand according to general formula (II) and at least one metal or radiometal.

Of particular interest is furthermore the labelling of the ligands and amide derivatives with radioisotopes, where in this context, the term labelling is referring to binding of the radioactive metal ions by the chelator by means of complex formation. Suitable radioisotopes include ⁴⁴Sc, ⁴⁶Sc, ⁵⁵Co, ^(99m)Tc, ²⁰³Pb, ⁶⁶Ga, ⁶⁷Ga, ⁶⁸Ga, ⁷²As, ¹¹¹In, ^(113m)In, ^(114m)In, ⁹⁷Ru, ⁶²Cu, ⁶⁴Cu, ⁵²Fe, ^(52m)Mn, ⁵¹Cr, ¹⁸⁶Re, ¹⁸⁸Re, ⁷⁷As, ⁹⁰Y, ⁶⁷Cu, ¹⁶⁹Er, ^(117m)Sn, ¹²¹Sn, ¹²⁷Te, ¹⁴²Pr, ¹⁴³Pr, ¹⁹⁸Au, ¹⁹⁹Au, ¹⁴⁹Tb, ¹⁶¹Tb, ¹⁰⁹Pd, ¹⁶⁵Dy, ¹⁴⁹Pm, ⁵¹Pm, ¹⁵³Sm, ¹⁵⁷Gd, ¹⁶⁶Ho, ¹⁷²Tm, ¹⁶⁹Yb, ¹⁷⁵Yb, ¹⁷⁷Lu, ¹⁰⁵Rh, and ¹¹¹Ag, preferably ⁴⁴Sc, ^(99m)Tc, ⁶⁷Ga, ⁶⁸Ga, ¹¹¹In, ⁶⁴Cu, ¹⁸⁸Re, ⁹⁰Y, ¹⁷⁷Lu.

The present invention further relates to a process for the preparation of a chelate according to the present invention, wherein at least one ligand according to general formula (II) and at least one metal and/or at least one metal comprising precursor are reacted.

The present invention further relates to the method of using chelate ligands or chelates according to the present invention in molecular imaging, preferably in multimodal molecular imaging. Examples of molecular imaging are positron emission tomography (PET), single photon emission computed tomography (SPECT), planar scintigraphy, fluorescence imaging, magnet resonance tomography (MRT), optical imaging, either fluorescence or bioluminescence imaging and combinations thereof, i.e. PET/CT and PET/MRT.

In a preferred embodiment of the present invention, the compounds according to general formula (II) can be divided into compounds according to general formula (IIa), (IIb) and (IIc) as shown in the following.

In a preferred embodiment of compounds according to the present invention radicals R¹, R² and/or R³ are connected to the core molecule via at least one linking group (LINKER). The preferred embodiment that all groups Z¹, Z² and Z³ are different from OH is, for example, shown in general formula (IIa)

wherein R¹, R², R³, R⁴, R⁵ and R⁶ have the same meanings as mentioned above. In this embodiment of the present invention, not all radicals R¹, R² and R³ have to be connected via a linking group. It is also possible that only one or two of R¹, R² and R³ are connected via at least one linking group, whereas the remaining radicals are directly attached to the core molecule. In addition, also in case, wherein not all groups Z¹, Z², Z³ are different from OH, linking groups my be present between R¹, R² or R³ and the core molecule, as defined above.

In a further preferred embodiment of compounds according to the present invention radicals R¹, R² and/or R³ are connected to the core molecule via at least one linking group (LINKER) and R¹, R² and/or R³ are so called biomolecules, the preferred embodiment that all groups Z¹, Z² and Z³ are different from OH is, for example, shown in general formula (IIb). It is also possible that none, only one or two of so called biomolecules are connected via at least one linking group, whereas the remaining radicals are directly attached to the core molecule.

In addition, also in case, wherein not all groups Z¹, Z², Z³ are different from OH, linking groups may be present between R¹, R² or R³ and the core molecule, as defined above.

In a further preferred embodiment of the compounds according to the present invention, radicals R¹, R² and/or R³ are so called biomolecules and/or additional signalling units, the preferred embodiments that all groups Z¹, Z² and Z³ are different from OH are, for example, shown in general formulae (IIc)

In this embodiment of the present invention, additional signalling units and/or biomolecule radicals may also be connected via a linking group. In addition, also in case, wherein not all groups Z¹, Z², Z³ are different from OH, linking groups may be present between R¹, R² or R³ and the core molecule, as defined above.

Additional signalling units can include chromophores, fluorophores, magnet resonance imaging (MRI) contrast agents such as chemical exchange saturation transfer (CEST) or paramagnetic chemical exchange saturation transfer (PARACEST) agents.

Compounds according to the present invention, where linking groups (LINKERS) are present, additional functional groups, for example carboxylic acid, amine, terminal alkyne, are present that allow for further functionalization of the linker groups on the other end.

According to the present invention, core molecule PrP9 (I) and the corresponding linker conjugates are further functionalized with biomolecules, e.g. the cyclic pentapeptide moiety cyclo(RGDfK). (see experimental section). In this way, precursors for radio tracers suitable for imaging of integrin expression are obtained, see for example general formula (IIa).

Also, derivatives of core molecule PrP9 (I) bearing biomolecules and additional reporter molecules, e.g. fluorophores, can be synthesized, see general formula (IIc), giving rise to tracers for application in multimodal imaging, e.g., the simultaneous application of nuclear and fluorescence imaging methods.

In general formula (II), R¹, R² and R³ are preferably identical. Furthermore, R¹, R² and/or R³ preferably comprise 1 to 200 carbon atoms, particularly preferably 1 to 100 carbon atoms.

R⁴, R⁵ and/or R⁶ are preferably hydrogen.

In case that R⁴, R⁵ and/or R⁶ are not hydrogen, they may in general comprise 1 to 200 carbon atoms, particularly preferably 1 to 100 carbon atoms.

In a further preferred embodiment, R¹, R² and R³ are independently of another based on amines being selected from the group consisting of cyclic, aliphatic amines, amino acids esters, amino acid esters, biotin, aliphatic phosphonates, peptides, proteins, residues thereof, antibodies, antibody fragments and engineered antibody formats, anticalines, biomolecules that bind with high affinity (low nM affinity) to other proteins, receptors, transporters of other molecular targets in vivo and in vitro, biomolecules, fluorophores and mixtures thereof.

In a preferred embodiment, R¹, R² and R³ are independently of another not based on amines being selected from the group consisting of cyclohexyl-amine, methyl-glycine, benzyl-glycine, tert-butyl-L-phenyl-alanine and mixtures thereof.

The present invention therefore preferably relates to chelate ligands according to the present invention, wherein chelate ligands having R¹, R² and R³ that are independently of another based on amines being selected from the group consisting of cyclohexyl-amine, methyl-glycine, benzyl-glycine, tert-butyl-L-phenyl-alanine and mixtures thereof, are excluded.

In a further preferred embodiment, R¹, R² and R³ are independently of another biomolecules selected from the group consisting of c(RGDfK)(Pbf,tBu), c(DGRKf)(Pbf,tBu), cyclo(D-Tyr¹-D-Orn²-Arg³-NaI⁴-Gly⁵) linked via D-Orn² (CPCR⁴), H-D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-ol (Disulfide bridge: 2-7), linked via D-Phe¹, H-D-Phe-Cys-Phe-D-Trp-Lys-Thr-Cys-Thr-OH (Disulfide bridge: 2-7), linked via D-Phe¹, H-D-Phe-Cys-BzThi³-D-Trp-Lys-Thr-Cys-Thr-OH (Disulfide bridge: 2-7, linked via D-Phe¹, H-D-Phe-Cys-BzThi³-D-Trp-Lys-Thr-Cys-Thr-ol (Disulfide bridge: 2-7), linked via D-Phe¹, linked via D-Phe¹, H-D-Phe-Cys-1-NaI³-D-Trp-Lys-Thr-Cys-Thr-OH (Disulfide bridge: 2-7), linked via D-Phe¹, H-D-Phe-Cys-1-NaI³-D-Trp-Lys-Thr-Cys-Thr-ol (Disulfide bridge: 2-7), [Lys⁴⁰(Ahx-DTPA)NH₂]-exendin-4, [Lys⁴⁰(Ahx-DOTA)NH2]-exendin-4, [Lys⁴,Phe⁷,Pro³⁴]NPY (linked via Lys⁴) Demobesin 1, panbombesin, minigastrin 11 and 9, Demogastrin, CCK8 (nonsulfated), pan-somatostatin (KE-88) and mixtures thereof.

In a further preferred embodiment, R¹, R² and R³ are independently of another selected from fluorophores selected from the group consisting of derivates of acridine, derivates of anthraquinone, arylmethane dyes, diarylmethane dyes, triarylmethane dyes, azo dyes, diazonium dyes, nitro dyes, nitroso dyes, derivates of phthalocyanine, derivates of quinone, azin dyes, eurhodin dyes, safranin dyes, indamins, indophenol dyes, oxazin dyes, oxazone dyes, thiazin dyes, thiazole dyes, derivates of thiazole, xanthene dyes, fluorene dyes, pyronin dyes, fluorone dyes, Rhodamine dyes, Porphyrine dyes, Cyanine dyes (Merocyanine, Indocyanine), coumarine dyes, stilbene derivatives, anthracene derivatives, styrylpyridinium dye, naphthalimid derivatives, squarine dyes, carbazoles, perylene derivatives, pyrene/benzopyrene derivatives, indol derivatives (tryptophan etc.), flavones, quinolinium dyes, pyridin derivatives, BODIPY dyes, ethidium dyes, naphthalene derivatives, ruthenium complexes, e.g. Ru bipyridine, Pd porphyrin complexes and mixtures thereof.

In a further preferred embodiment, R¹, R² and R³ are independently of another selected from CEST or PARACEST agents selected from the group of lanthanoid metal ion chelates, particularly those based on azamacrocyclic chelating units, such as DOTA complexes.

In a further preferred embodiment, linking groups (LINKER) that attach radicals R¹, R² and/or R³ to the core molecule (I) are independently of another selected from natural and unnatural amin acids, like glutamic acid, aspartic acid, lysine, glycine, alanine, proline, serine, threonine, phenylalanine, tyrosine, to mention only a few, peptide linkers, amino acid esters, difunctional molecules, such as co-amino-carboxylic acids (H₂N—(CH₂)_(n)—COOH, n=1−12) or polyalkyleneoxide based molecules, e.g. polyethyleneoxide based amino carboxylic acid esters, and mixtures thereof.

Particularly preferred radicals R¹, R², R³, linking groups, biomolecules, fluorophores according to the present invention are shown in the examples.

Particularly preferred chelate ligands according to the present invention have the following formulae

The present invention further relates to a process for the preparation of chelate ligands according to general formula (II) by reaction of chelate ligands of formula (I)

with R¹R⁴NH, R²R⁵NH, R³R⁶NH, wherein R¹, R², R³, R⁴, R⁵, R⁶ have the same meanings as to mentioned above.

PrP9 according to compound formula (I) bears three carboxylic acid groups which allow for functionalization by amide formation with e.g. amines comprising R¹, R², R³, R⁴, R⁵ and/or R⁶ as mentioned above, biomolecules and/or fluorophores, optionally attached via linking groups, in order to obtain radiotracers.

Coupling of PrP9 according to formula (I) and suitable amines can in general be conducted according to methods known to the skilled artisan and described for example in Chem. Eur. J. 2010, 16, 7174-7185.

The process according to the present invention can be conducted in any suitable solvent, for example dimethylformamide (DMF), dimethyl sulfoxide (DMSO), dimethylacetamide (DMA), N-methylpyrrolidinone (NMP) or mixtures thereof.

The process according to the present invention can be conducted using any suitable additive that is known to the skilled artisan for supporting amide coupling reactions, for example di-iso-propyl-ethyl-amine (DIPEA), triethylamine (TEA), 1,8-Diazabicyclo[5.4.0]undec-7-en (DBU), 1,5-Diazabicyclo[4.3.0]non-5-ene (DBN), or mixtures thereof.

The process according to the present invention can be conducted using any suitable coupling agent, for example 2-(1H-7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate methanaminium (HATU), 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU), 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (HBTU), O-(benzotriazol-1-yl)-N,N,N,N′-tetramethyluronium tetrafluoroborate (TBTU), benzotriazole-1-yl-oxy-tris-(dimethylamino)phosphonium hexafluorophosphate (BOP), (benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyBOP), 3(ethyliminomethyleneamino)-N,N-dimethyl-propan-1-amine (EDC), N,N′-dicyclohexylcarbodiimide (DCC), N,N′-diisopropylcarbodiimide (DIC), 3-(diethoxyphosphoryloxy)-3H-benzo[d][1,2,3]triazin-4-one (DEPBT), N,N′-carbonyldiimidazole (CDI), O-(6-chloro-1-hydroxybenzotriazotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TCTU), O-(7-azabenzotriazole-1-yl)-N,N,N′N′-tetramethyluronium tetrafluoroborate (TATU), bromo-tris-pyrrolidinophosphonium hexafluorophosphate (PyBrOP), 2-(6-chloro-1H-benzotriazole-1-yl)-1,1,3,3-tetramethylaminium hexafluorophosphate) or mixtures thereof.

In respect of amines R¹R⁴NH, R²R⁵NH and/or R³R⁶NH, wherein R¹, R², R³, R⁴, R⁵, R⁶ have the same meanings as mentioned above. Furthermore, the specific definitions that have been made according to general formula (II), also apply here.

The present invention further relates to a chelate comprising at least one chelate ligand according to general formula (II) as defined above and at least one metal or radiometal.

In a preferred embodiment, the present invention relates to a chelate according to the present invention, wherein the at least one metal or radiometal is selected from the group consisting of La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu²⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺, Sc³⁺, Y³⁺, Al³⁺, Ga³⁺, Ge⁴⁺, In³⁺, As³⁺, Sn²⁺, Sn⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Cu¹⁺, Cu²⁺, Zn²⁺, Cd²⁺ and mixtures thereof.

In a particular preferred embodiment, the present invention relates to a chelate according to the present invention, wherein the at least one radiometal is selected from the group consisting of ⁴⁴Sc, ⁴⁶Sc, ⁵⁵Co, ^(99m)Tc, ²⁰³Pb, ⁶⁶Ga, ⁶⁷Ga, ⁶⁸Ga, ⁷²As, ¹¹¹In, ^(113m)In, ^(114m)In, ⁹⁷Ru, ⁶²Cu, ⁶⁴Cu, ⁵²Fe, ^(52m)Mn, ⁵¹Cr, ¹⁸⁶Re, ¹⁸⁸Re, ⁷⁷As, ⁹⁰Y, ⁶⁷Cu, ¹⁶⁹Er, ^(117m)Sn, ¹²¹Sn, ¹²⁷Te, ¹⁴²Pr, ¹⁴³Pr, ¹⁹⁸Au, ¹⁹⁹Au, ¹⁴⁹Tb, ¹⁶¹Tb, ¹⁰⁹Pd, ¹⁶⁵Dy, ¹⁴⁹Pm, ¹⁵¹Pm, ¹⁵³Sm, ¹⁵⁷Gd, ¹⁶⁶Ho, ¹⁷²Tm, ¹⁶⁹Yb, ¹⁷⁵Yb, ¹⁷⁷Lu, ¹⁰⁵Rh, ¹¹¹Ag and mixtures thereof, preferably selected from the group consisting of ⁴⁴Sc, ^(99m)Tc, ⁶⁷Ga, ⁶⁸Ga, ¹¹¹In, ⁶⁴Cu, ¹⁸⁸Re, ⁹⁰Y, ¹⁷⁷Lu and mixtures thereof. In respect of the mentioned radiometals, preferably the oxidation state as mentioned above is present.

A particularly preferred metal is Ga, in particular ⁶⁸Ga, very particularly ⁶⁸Ga³⁺. Nuclear metal atoms and ions are prepared according to methods known to the skilled artisan, for example by generator based preparation. Suitable compounds comprising the mentioned metal ions are for example aqueous or other solutions of metal salts, such as clorides, nitrates, sulfates, phosphates, carbonates, hydrogencarbonates.

In a preferred embodiment of the chelates according to the present invention, R¹, R² and R³ are independently of another not based on amines being selected from the group consisting of cyclohexyl-amine, methyl-glycine, benzyl-glycine, tert-butyl-L-phenyl-alanine and mixtures thereof.

The present invention therefore preferably relates to chelates according to the present invention, wherein chelates having R¹, R² and R³ that are independently of another based on amines being selected from the group consisting of cyclohexyl-amine, methyl-glycine, benzyl-glycine, tert-butyl-L-phenyl-alanine and mixtures thereof, are excluded.

Complexation with Metals and Complexation with Radiometals

The present invention further relates to a process for the preparation of a chelate according to the present invention, wherein at least one residue R¹, R², R³ according to general formula (II) and the PrP9 core is labelled with at least one metal or radiometal.

Preferably, the present invention relates to the process according to the present invention, wherein the at least one metal or radiometal is selected from the group consisting of La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu²⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺, Sc³⁺, Y³⁺, Al³⁺, Ga³⁺, Ge⁴⁺, In³⁺, As³⁺, Sn²⁺, Sn⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Cu¹⁺, Cu²⁺, Zn²⁺, Cd²⁺ and mixtures thereof.

Particularly preferably, the present invention relates to the process according to the present invention, wherein the at least one radiometal is selected from the group consisting of ⁴⁴Sc, ⁴⁶Sc, ⁵⁵Co, ^(99m)Tc, ²⁰³Pb, ⁶⁶Ga, ⁶⁷Ga, ⁶⁸Ga, ⁷²As, ¹¹¹In, ^(113m)In, ^(14m)In, ⁹⁷Ru, ⁶²Cu, ⁶⁴Cu, ⁵²Fe, ^(52m)Mn, ⁵¹Cr, ¹⁸⁶Re, ¹⁸⁸Re, ⁷⁷As, ⁹⁰Y, ⁶⁷Cu, ¹⁶⁹Er, ^(117m)Sn, ¹²¹Sn, ¹²⁷Te, ¹⁴²Pr, ¹⁴³Pr, ¹⁹⁸Au, ¹⁹⁹Au, ¹⁴⁹Tb, ¹⁶¹Tb, ¹⁰⁹Pd, ¹⁶⁵Dy, ¹⁴⁹Pm, ¹⁵¹Pm, ¹⁵³Sm, ¹⁵⁷Gd, ¹⁶⁶Ho, ¹⁷²Tm, ¹⁶⁹Yb, ¹⁷⁵Yb, ¹⁷⁷Lu, ¹⁰⁵Rh, ¹¹¹Ag and mixtures thereof, preferably selected from the group consisting of ⁴⁴Sc, ^(99m)Tc, ⁶⁷Ga, ⁶⁸Ga, ¹¹¹In, ⁶⁴Cu, ¹⁸⁸Re, ⁹⁰Y, ¹⁷⁷Lu and mixtures thereof. In respect of the mentioned radiometals, preferably the oxidation state as mentioned above is present.

For example, the ⁶⁸Ga labelling of a symmetrical c(RGDfK)-PEG4-conjugate of PrP9, R3PEG4P9 (see below), has been performed with generator-produced ⁶⁸Ga using an automated system for labelling (for details see experimental section). On the same system, peptide conjugates of the competing chelators DOTA and NODAGA, namely DOTATOC and NODAGA-RGD (see figure) have been labelled.

Preclinical Studies

Due to the high specific activity, tracers, being chelates comprising chelate ligands according to the present invention and metals obtained as described above can be used directly for preclinical studies using small animals, without the necessity of prior separation of the labelled compound from the precursor, i.e., the functionalized chelator without radiometal ion. This is of importance because in small animals such as rodents, the absolute molar amount of injected tracer will affect the result of the study, particularly in case the addressed receptor/tissue is saturable.

As an example, the PEG4-linked cyclo(RGDfK) conjugate R3PEG4P9 was evaluated in mice with tumor xenografts (see Experimental section). Biodistribution studies as well as PET imaging showed the suitability of the tracer for imaging of integrin expression. Moreover, the analysis of metabolites proved that the compound is fully stable in vivo.

DESCRIPTION OF THE FIGURES

FIG. 1 shows product yields for the preparation of ⁶⁸Ga-R3PEG4P9 (solid line), ⁶⁸Ga-DOTATOC (dashed line) and ⁶⁸Ga-NODAGA-RGD (semi-transparent line), are given as functions of precursor amount.

FIG. 2 shows the specific activities for preparations of ⁶⁸Ga-R3PEG4P9 (solid line), ⁶⁸Ga-DOTATOC (dashed lines) and ⁶⁸Ga-NODAGA-RGD (semi-transparent lines) 30 min after the start of the synthesis (t=30 min). The initial ⁶⁸Ga activity (t=0) was 15 mCi (555 MBq). (In practice, slightly varying ⁶⁸Ga amounts (400-600 MBq) were used; the data thus obtained were normalized.)

The curves show that in comparison, the amount of R3PEG4P9 necessary for high labelling yields is lower than in case of DOATOC and NODAGA-RGD, thus increasing the maximal achievable specific activity (here: >4 TBq/μmol). At optimal pH conditions for labelling of the latter compounds (3.2-3.3, see experimental section), the maximum specific activity of DOTATOC is limited to <400 GBq/μmol and of NODAGA-RGD to <600 GBq/μmol. In contrast, R3PEG4P9 can easily be labelled at pH 1.8, reaching specific activities above 1000 GBq/μmol and up to 5000 GBq/μmol.

FIG. 3: Data from the table under the headline “Biodistribution of ⁶⁸Ga-R3PEG4P9” is visualized in the chart.

FIGS. 4 a-h: HPLC chromatograms of ⁶⁸Ga-R3PEG4P9 (for reference) and the analyzed extracts referred to under the headline “Metabolite studies for ⁶⁸Ga-R3PEG4P9” are shown, indicating that no metabolites could be detected.

FIG. 5: The graphic shows PET images derived from data measured 60-90 min after injection (maximum intensity projections; left: R3PEG4P9, center: R3PEG4P9 with blockade, right: isoR3PEG4P9).

FIGS. 6 a-c: The charts (FIG. 6) show distribution kinetics derived from PET data.

EXPERIMENTAL & EXAMPLES General

Analytical HPLC was performed using a Sykam HPLC system with low-pressure gradient mixer, equipped with a Nucleosil C18-RP column (100×4.6 mm, 5 μm particle size), at a flow rate of 1 ml/min. Eluents were water and acetonitrile, both containing 0.1% trifluoroacetic acid (TFA). Two gradients were used: Gradient A, 20-80% MeCN in 24 min and Gradient B, 40-100% MeCN in 24 min.

Preparative HPLC was done using a Sykam system with two separate solvent pumps, equipped with a YMC C18ec column (250×30 mm, 5 μm particle size), at a flow rate of 20 ml/min. Solvents were similar to analytical HPLC. Separations were generally done in isocratic mode with eluent compositions individually optimized for each compound (see below). ESI−MS was measured on a Varian LC-MS system.

Peptide Synthesis

The protected cyclic pentapeptides (see scheme below) were prepared according to literature protocols, using standard methods for solid phase peptide synthesis (Fmoc strategy).

5-Azidopentanoic acid (28.6 mg) and DIPEA (65 mg, 85 μl) were dissolved in DMF (0.5 ml). Then HATU (84 mg) was added with stirring. After 5 min, cRGDfK(Pbf,tBu) (100 mg), dissolved in DMF (0.5 ml) was added. After 10 min, the product was precipitated by slow addition of the reaction mixture to 10 ml of brine, and centrifuged. The liquid was poured off, the solids redissolved in 0.2 ml DMF, and precipitated by addition of diethyl ether (10 ml). After centrifugation and removal of the ether phase the product was stirred in ethanol (1 ml) and again ether was added for precipitation. After another centrifugation, the solid was separated off and dried in vacuo. Yield: 77 mg.

General Synthetic Procedure for Coupling of Amines Using HATU

Molar amounts of reagents used are given below for each synthesis and individual compounds.

PrP9 or its derivatives with linkers, diisopropylethylamine (DIPEA) and the amine were dissolved in DMSO. Then HATU was added with stirring. RP-HPLC was used for reaction control. After the reaction had finished, workup was performed according to solubility of the products. In case of water-soluble compounds, the reaction mixtures were diluted with water and subjected to diafiltration with 0.05 M NaCl solution and then pure water (Amicon stirred cell, membrane with 0.5 kDa MWCO), followed by lyophilisation of the cell contents. Products not soluble in water were precipitated by slow addition of the reaction mixture to brine. The solids were separated by centrifugation and dried in vacuo.

In case some remaining impurities were detected in the crude product, further purification was done using preparative HPLC. Following this step, the organic part of the eluent was removed in vacuo and the remaining aqueous solutions lyophilized to yield the final products.

General Procedure for Removal of Acid-Sensitive Protecting Groups (Pbf, tBu)

The dry compound was dissolved in 0.5-1 ml trifluoroacetic acid and allowed to react for 24 h. Then, the mixture was slowly added to diethyl ether, the precipitate separated by centrifugation and dried in vacuo. If necessary, purification was done using preparative HPLC, followed by concentration of the eluates and lyophilization to yield the final products.

Syntheses

General coupling protocol using PrP9 (62 mg), DMSO (1 ml), DIPEA (258 mg, 340 μl), cRGDfK(Pbf,tBu) (100 mg), HATU (305 mg).

Reaction time: 10 min. t_(R) of R1P9 (Gradient B): 8.5 min; t_(R) of R2P9 (Gradient B): 14 min. Workup: Ultrafiltration. Purification by preparative HPLC (two-stage isocratic elution: 47% MeCN for 10 min, followed by 63% MeCN for another 10 min, thus providing purified pR1P9 and pR2P9 in one separation run).

Yield: 39 mg of pR1P9 and 22 mg of pR2P9.

Acidic deprotection directly yielded the compounds R1P9 and R2P9 in quantitative yield.

General coupling protocol using PrP9 (512 mg), DMSO (12 ml), DIPEA (1560 mg, 2 ml), benzylglutamic acid tert-butyl ester hydrochloride (1320 mg), HATU (2 g).

Reaction time: 24 h. Workup: Precipitation, then purification by preparative HPLC (80% MeCN, t_(R)=10 min). Collected eluates were concentrated, neutralized by addition of aq. Na—HCO3, the salts precipitated by addition of tert-butanol, and lyophilized. Yield: 880 mg. Debenzylation was done by stirring 220 mg of BtGlutP9 in 5 ml methanol with 50 mg Pd/C under hydrogen atmosphere for 3 h. The mixture was filtered through celite and the solvent removed in vacuo. Yield: 115 mg.

General coupling protocol using tGlutP9 (20 mg) DMSO (0.5 ml), DIPEA (41 mg, 54 μl), cRGDfK(Pbf,tBu) (80 mg), HATU (49 mg).

Reaction time: 10 min. t_(R) (Gradient B): 20 min. Workup: Precipitation, followed by acidic deprotection. Purification: preparative HPLC (22% MeCN, t_(R) ca. 15 min). Yield: 28 mg.

General coupling protocol using PrP9 (308 mg), DIPEA (967 mg, 1280 μl), DMSO (5 ml), methyl-6-aminohexanoic acid hydrochloride (455 mg), HATU (1.52 g).

Reaction time: 10 min. t_(R) (Gradient A): 11.5 min. Workup: Ultrafiltration. Deprotection was done by dissolving the methyl ester in a mixture of water (2 ml) and methanol (6 ml) MeOH containing LiOH (100 mg) and allowing to react at 4° C. for 72 h. t_(R) (Gradient A): ca. 7 min. Purification by preparative HPLC (19% MeCN, t_(R) ca. 14 min). Yield: 183 mg.

General coupling protocol using HexP9 (16 mg) DMSO (0.7 ml), DIPEA (39 mg, 51 μl) cRGDfK(Pbf,tBu) (80 mg), HATU (46 mg). Reaction time: 10 min. t_(R) (Gradient B): 11.4 min. Workup: Precipitation, followed by acidic deprotection. Purification by preparative HPLC (26% MeCN, t_(R) ca. 15 min). Yield: 23 mg.

General coupling protocol using PrP9 (246 mg), DIPEA (516 mg, 680 μl), DMSO (4 ml), H₂N-dPEG™ (4)-COOtBu (from Iris Biotech, Marktredwitz, Germany; 455 mg), HATU (1.22 g). Reaction time: 10 min. t_(R) (Gradient A): 16 min. Workup: Ultrafiltration, followed by acidic deprotection. Purification by preparative HPLC (21% MeCN, t_(R) ca. 12 min). Yield: 410 mg.

General coupling protocol using PEG4P9 (21 mg), DMSO (0.3 ml), DIPEA (36 mg, 48 μl), CPCR4 (50 mg), HATU (43 mg). Reaction time: 10 min. t_(R) (Gradient A): 6.7 min. Purification: Precipitation, followed by preparative HPLC (46% MeCN, t_(R) ca. 13 min). Yield: 23 mg.

General coupling protocol using PEG4P9 (16 mg) DMSO (0.7 ml), DIPEA (31 mg, 41 μl) cDGRKf(Pbf,tBu) (60 mg), HATU (37 mg). Reaction time: 10 min. t_(R) (Gradient B): 10.7 min. Workup: Precipitation, followed by acidic deprotection. Purification by preparative HPLC (25% MeCN, t_(R) ca. 14 min). Yield: 11 mg.

General coupling protocol using PEG4P9 (24 mg) DMSO (0.7 ml), DIPEA (39 mg, 51 μl) cRGDfK(Pbf,tBu) (80 mg), HATU (46 mg). Reaction time: 10 min. t_(R) (Gradient B): 10.3 min. Workup: Precipitation, followed by acidic deprotection. Purification by preparative HPLC (27% MeCN, t_(R) ca. 15 min). Yield: 28 mg.

General coupling protocol using PrP9 (246 mg), DIPEA (516 mg, 680 μl), DMSO (4 ml), H₂N-dPEG™ (8)-COOtBu (from Iris Biotech, Marktredwitz, Germany; 994 mg), HATU (1.22 g). Reaction time: 10 min. t_(R) (Gradient A): 15 min. Workup: Ultrafiltration, followed by acidic deprotection. Purification by preparative HPLC (22% MeCN, t_(R) ca. 12 min). Yield: 539 mg.

General coupling protocol using PEG8P9 (27 mg), DMSO (0.5 ml), DIPEA (36 mg, 48 μl), CPCR4 (50 mg), HATU (43 mg). Reaction time: 10 min. t_(R) (Gradient A):

Purification: Precipitation, followed by preparative HPLC (49% MeCN, t_(R) ca. 12 min). Yield: 18 mg.

General coupling protocol using PEG8P9 (12 mg) DMSO (0.7 ml), DIPEA (15 mg, 20 μl) cDGRKf(Pbf,tBu) (30 mg), HATU (19 mg). Reaction time: 10 min. t_(R) (Gradient B): 11.1 min. Workup: Precipitation, followed by acidic deprotection. Purification by preparative HPLC (31% MeCN, t_(R) ca. 15 min). Yield: 7 mg.

General coupling protocol using PEG8P9 (32 mg) DMSO (0.7 ml), DIPEA (41 mg, 54 μl) cRGDfK(Pbf,tBu) (80 mg), HATU (49 mg). Reaction time: 10 min. t_(R) (Gradient B): 11.2 min.

Workup: Precipitation, followed by acidic deprotection. Purification by preparative HPLC (30% MeCN, t_(R) ca. 16 min). Yield: 22 mg.

General coupling protocol using PrP9 (308 mg), DMSO (3 ml), DIPEA (645 mg, 850 μl), propargylamine (138 mg, 172 μl), HATU (1.5 g). Reaction time: 15 min. Workup: Ultrafiltration; no further purification necessary. Yield: 354 mg.

ProP9 (40 mg) and sodium ascorbate (100 mg) were dissolved in water (0.5 ml). Then a solution of benzyl azide (20 mg, 19 μl) in 0.5 ml MeOH was added. Upon addition of a solution of Cu(OAc)₂*H₂O (12 mg) in water (0.5 ml) a clear green reaction mixture was obtained. Purification was done by ultrafiltration. After concentration of the UF cell contents to 10 ml, solid Na₂S (50 mg) was added, whereupon a black precipitate was formed. Filtration over celite yielded a clear brown solution, presumably due to remaining colloidal sulfide precipitate. Purification was done by preparative HPLC. Yield: 25 mg.

ProP9 (12 mg) and sodium ascorbate (30 mg) were dissolved in methanol (0.2 ml). Then a solution of azido-cRGDfK(Pbf,tBu) (50 mg) in DMF (0.5 ml) was added. The mixture turned brown when a suspension of Cu(OAc)₂*H₂O (12 mg) in MeOH (0.5 ml) was added. After 70 min the product was precipitated by slow addition of the reaction mixture to 10 ml of brine, the precipitate separated by centrifugation and dried in vacuo. After acidic deprotection a light-green solid was obtained, which was dissolved in methanol/water mixture. Upon addition of sodium sulfide (50 mg) a brown precipitate was formed, which was filtered off. The filtrate was evaporated and the crude product purified by preparative HPLC (27% MeCN, t_(R) ca. 11 min). Yield: 11 mg.

General coupling protocol using pR2P9 (15 mg), DMF (0.2 ml), DIPEA (8.5 mg, 11 μl) and aminohexyl-rhodamine-6G*2TFA (from Sigma, 10 mg) and HATU (13 mg). Reaction time: 10 min. t_(R) (Gradient B): 19.5 min. Workup: Precipitation, followed by acidic deprotection. Purification by preparative HPLC (38% MeCN, t_(R) ca. 18 min.). Yield: 10 mg.

⁶⁸Ga Labelling

⁶⁸Ga-labelling was performed on an automated system (GallElute+ module from Scintomics, Fürstenfeldbruck, Germany), carrying out the following steps. ⁶⁸Ga was obtained from a generator with SnO₂ matrix (manufactured by IThembaLABS, South Africa, distributed by IDB Holland) which was eluted with 1.0 M HCl.

The precursor (e.g. R3PEG4P9, different molar amounts) was placed in a 4 ml conical reaction vial (AllTech), together with a solution of 260 mg 2-(4-(2-Hydroxyethyl)-1-piperazinyl)-ethansulfonsäure (HEPES) in 220 μl Wasser. Then a 1.25 ml fraction of the generator eluate, containing the hightest activity (between 500 and 600 MBq) was added, resulting in a pH of 1.8. The vial was heated to 100° C. for 5 min. Then the reaction mixture was passed over a SPE cartridge (Waters SepPak Classic C18), the cartridge purged with 10 ml of water to remove free ⁶⁸Ga³⁺, inorganic ions and HEPES, and purged with air. The product was eluted with 2 ml of a 1:1 mixture of ethanol and water and the cartridge and lines purged with 1 ml of water. For animal experiments, 1 ml of PBS (pH 7.4) was added and the solution concentrated in vacuo to 1 ml, thus removing all ethanol and producing a formulation suitable for injection.

For comparison purposes, the labeling of DOTATOC and NODAGA-RGD was carried out in exactly the same way as described above, only using 600 mg HEPES in 500 ml water to achieve a labelling pH of 3.2-3.3 which is ideal for these compounds.

Log P Determination

Octanol-water partition coefficients were determined by addition of ca. 50 kBq of the respective labelled compound to Eppendorf cup containing each 500 μl of 1-octanol and isotonic phosphate buffered saline (PBS). After 2 min of vigorous stirring, the phases were separated by centrifugation, 100 μl aliquots of each phase taken out and the activity contained determined with a gamma counter. Each experiment was repeated 5-8 times.

In Vitro Binding Assay

Binding assays were done for all RGD conjugates, their natGa complexes, and also for echistatin and 19F-galacto-RGD to act as standards.

Determination of integrin receptor affinity was carried out using M21 human melanoma cells, possessing high α_(v)β₃ expression. Experiments were carried out in 24-well plates. Ca. 2×10⁵ cells were seeded into wells containing RPMI 1640 media and incubated for 24 h at 37° C. and 5% CO₂. Then the medium was exchanged with 0.5 ml, binding buffer (20 mmol/l Tris, pH 7.4, 150 mmol/l NaCl, 2 mmol/l CaCl₂*2H₂O, 1 mmol/l MgCl₂*6H₂O, 1 mmol/l MnCl₂*4H₂O, 0.1% (m/m) BSA), containing 30.000-50.000 cpm ¹²⁵I-echistatin and the respective RGD peptide conjugate in increasing concentrations from 10⁻¹¹-10⁻⁴ M. After incubation at room temperature for 2 h, the supernatant is removed, the cells washed twice with PBS, lysed with 1 M NaOH (1 ml) and the lysates counted for 60 s in a gamma counter. Experiments were performed at minimum three times in duplicates and IC₅₀ values calculated using GraphPad prism for sigmoidal (dose-response) regression analysis.

In the table below, logarithmic IC₅₀ values with logarithmic errors, corresponding IC₅₀ values, and log P values for the standard, all RGD conjugates and their Ga(III)-complexes are given (see Synthesis section).

IC₅₀ Compound log IC₅₀ (nM) logP Echistatin −9.01 ± 0.033 0.98 — 19F-Galacto-RGD −6.45 ± 0.157 319 — R3PEG4P9 −7.37 ± 0.053 43 — Ga-R3PEG4P9 −7.36 ± 0.049 44 −3.90 ± 0.10 R3PEG8P9 −7.30 ± 0.029 50 — Ga-R3PEG8P9 −7.13 ± 0.034 74 −4.13 ± 0.16 R3GlutP9 −6.90 ± 0.069 125 — Ga-R3GlutP9 −6.66 ± 0.068 220 −4.03 ± 0.12 R3HexP9 −7.07 ± 0.075 85 — Ga-R3HexP9 −7.25 ± 0.080 56 −3.99 ± 0.13 R1P9 −5.53 ± 0.047 2890 — Ga-R1P9 −5.64 ± 0.058 2270 −3.91 ± 0.11 R2P9 −6.43 ± 0.128 370 — Ga-R2P9 −6.29 ± 0.107 512 −4.12 ± 0.06 R2RhoP9 −7.04 ± 0.031 92 — Ga-R2RhoP9 −7.11 ± 0.043 78 −1.27 ± 0.04 Biodistribution of ⁶⁸Ga-R3PEG4P9

Biodistribution studies were performed using CD-1 athymic nude mice bearing human melanoma xenografts on both shoulders (right: M21 cell line with high α_(v)β₃ integrin xexpression, left: M21L cell line with low α_(v)β₃ integrin expression). The mice were injected 6-9 MBq of ⁶⁸Ga-R3PEG4P9 (specific activity: ca. 900 GBq/μmol, corresponding to approx. 7-10 pmol of the tracer). After the specified time points (60 and 120 min, respectively), the mice were sacrificed, the organs taken out and counted in a gamma counter. For blockade, the mice were administered 200 μg (approx. 10 mg/kg) of unlabeled R3PEG4P9 10 min before tracer injection.

The table shows uptake values (given as percent injected dose per gram tissue) 60 and 120 min after tracer injection, as well as 60 min after injection with blockade. Data is visualized in the chart (FIG. 3).

% ID/g % ID/g % ID/g time 60 min 120 min 60 min, blocked number of animals 5 5 3 blood 0.31 0.16 0.09 heart 1.29 1.47 0.08 lung 2.96 2.15 0.54 liver 5.18 3.97 0.32 spleen 3.38 3.50 0.24 pancreas 0.88 0.81 0.07 stomach 4.43 3.56 0.17 small int. 4.92 3.75 0.24 large int. 2.44 2.27 0.11 kidneys 10.16 8.63 1.97 adrenal gland 27.77 24.17 0.36 muscle 0.95 0.67 0.05 thyroid 3.27 2.89 0.17 tumor M21L 1.48 1.70 0.31 tumor M21 6.08 4.63 0.62 Metabolite Studies for ⁶⁸Ga-R3PEG4P9

Metabolite studies were performed using mice similar as used for biodistribution. The mice were anaesthesized with isoflurane and injected 30-40 MBq of ⁶⁸Ga-R3PEG4P9 (specific activities between 1200-3200 MBq/nmol, injected molar amounts ranging from 9-18 pmol). After 30 min the animals were sacrificed, the blood was collected in a heparine syringe and centrifuged. The respective organs were removed, frozen with liquid nitrogen and homogenized by means of a ball mill. The resulting powder was suspended in 0.5-1 mL of PBS, together with 30 μg of R3PEG4P9, stirred for 1 min, and centrifuged. For both organs and blood, the supernatant (plasma, respectively) was separated, both the pellet and supernatant counted in a gamma-counter in order to determine extraction efficiency or blood cell binding. Supernatants, plasma and urine were subjected to ultrafiltration (30 kDa MWCO) and analyzed by radio-HPLC (Merck chromolith column 100×4.6 mm; flux rate 2 ml/min; eluents: A, water with 0.1% TFA, B, acetonitrile with 0.1% TFA; isocratic elution for 2 min with 3% B, followed by gradient from 3-60% B in 6 min and purging with 95% B for 3 min). For determination of adrenal gland/pancreas/spleen metabolites, organs of three animals were pooled.

The table shows extraction efficiencies for the tissues worked up with the above described procedure; * the fraction of the blood activity in the plasma after centrifugation is given.

tissue extraction efficiency* kidney 92% tumor M21 92% liver 81% spleen/pancreas 88% adrenal gland 95% blood plasma 96%

In FIG. 4, HPLC chromatograms of ⁶⁸Ga-R3PEG4P9 (for reference) and the analyzed extracts are shown, indicating that no metabolites could be detected.

PET Imaging

Preclinical imaging was done with mice similar as used for biodistribution. The mice were anaesthesized with isoflurane, placed in the PET camera, and 13-14 MBq of ⁶⁸Ga-R3PEG4P9 (without blockade and with prior injection of 100 μg of unlabelled R3PEG4P9) or ⁶⁸Ga-isoR3PEG4P9 were administered via tail vein injection. The calculated molar amount of labelled tracer per injection was ca. 20 pmol with a calculated minimum specific activity of ca. 900 GBq/μmol. PET scans were recorded dynamically for 90 min.

The graphic (FIG. 5) shows PET images derived from data measured 60-90 min after injection (maximum intensity projections; left: R3PEG4P9, center: R3PEG4P9 with blockade, right: isoR3PEG4P9).

The charts (FIG. 6) show distribution kinetics derived from PET data.

The invention also refers to the following items:

-   -   (1) Chelate ligands according to general formula (II)

-   -    wherein         -   Z¹ is OH or NR¹R⁴,         -   Z² is OH or NR²R⁵,         -   Z³ is OH or NR³R⁶,         -   R¹, R², R³ is independently of another selected from the             group consisting of linear or cyclic, substituted or             unsubstituted, aliphatic, heteroaliphatic, aromatic,             heteroaromatic, saturated or unsaturated radicals, wherein             said R¹, R² and/or R³ is optionally attached to the core             molecule via at least one linking group,         -   R⁴, R⁵, R⁶ is independently of another selected from the             group consisting of hydrogen, linear or cyclic, substituted             or unsubstituted, aliphatic, heteroaliphatic, aromatic,             heteroaromatic, saturated or unsaturated radicals,         -   wherein at least one of Z¹, Z² and Z³ is different from OH.     -   (2) Chelate ligand according to item 1 wherein, R⁴, R⁵ and R⁶         are hydrogen.     -   (3) Chelate ligand according to item 1, wherein R¹, R² and R³         are independently of another based on amines being selected from         the group consisting of cyclic, aliphatic amines, amino acids         esters, amino acid esters, biotin, aliphatic phosphonates,         peptides, proteins, residues thereof, antibodies, antibody         fragments and engineered antibody formats, anticalines,         biomolecules that bind with high affinity (low nM affinity) to         other proteins, receptors, transporters of other molecular         targets in vivo and in vitro, biomolecules, fluorophores and         mixtures thereof.     -   (4) Chelate ligand according to any of items 1 to 3 having one         of the following formulae

-   -   (5) Process for the preparation of chelate ligands according to         any of items 1 to 4 by reaction of chelate ligands of formula         (I)

-   -    with R¹R⁴NH, R²R⁵NH, R³R⁶NH, wherein R¹, R², R³, R⁴, R⁵, R⁶         have the same meanings as defined in any of items 1 to 4.     -   (6) Chelate comprising at least one chelate ligand according to         general formula (II) as defined in any of items 1 to 4 and at         least one metal or radiometal.     -   (7) Chelate according to item 6, wherein the at least one metal         or radiometal is selected from the group consisting of La³⁺,         Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu²⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺,         Tm³⁺, Yb³⁺, Lu³⁺, Sc³⁺, Y³⁺, Al³⁺, Ga³⁺, Ge⁴⁺, In³⁺, As³⁺, Sn²⁺,         Sn⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Cu¹⁺,         Cu²⁺, Zn²⁺, Cd²⁺ and mixtures thereof.     -   (8) Chelate according to item 6 or 7, wherein the at least one         radiometal is selected from the group consisting of ⁴⁴Sc, ⁴⁶Sc,         ⁵⁵Co, ^(99m)Tc, ²⁰³Pb, ⁶⁶Ga, ⁶⁷Ga, ⁶⁸Ga, ⁷²As, ¹¹¹In, ^(113m)In,         ^(114m)In, ⁹⁷RU, ⁶²Cu ⁶⁴Cu, ⁵²Fe, ^(52m)Mn, ⁵¹Cr, ¹⁸⁶Re, ¹⁸⁸Re,         ⁷⁷As, ⁹⁰Y, ⁶⁷Cu, ¹⁶⁹Er, ^(117m)Sn, ¹²¹Sn, ¹²⁷Te, ¹⁴²Pr, ¹⁴³Pr,         ¹⁹⁸Au, ¹⁹⁹Au, ¹⁴⁹Tb, ¹⁶¹Tb, ¹⁰⁹Pd, ⁶⁵Dy, ¹⁴⁹Pm, ¹⁵¹Pm, ¹⁵³Sm,         ¹⁵⁷Gd, ⁶⁶Ho, ¹⁷²Tm, ¹⁶⁹Yb, ¹⁷⁶Yb, ¹⁷⁷Lu, ¹⁰⁵Rh, ¹¹¹Ag and         mixtures thereof, preferably selected from the group consisting         of ⁴⁴Sc, ^(99m)Tc, ⁶⁷Ga, ⁶⁸Ga, ¹¹¹In, ⁶⁴Cu, ¹⁸⁸Re, ⁹⁰Y, ¹⁷⁷Lu         and mixtures thereof.     -   (9) Process for the preparation of a chelate according to item         6, wherein at least one residue R¹, R², R³ according to general         formula (II) and the PrP9 core is labelled with at least one         metal or radiometal.     -   (10) Process according to item 9, wherein the at least one metal         or radiometal is selected from the group consisting of La³⁺,         Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu²⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺,         Tm³⁺, Yb³⁺, Lu³⁺, Sc³⁺, Y³⁺, Al³⁺, Ga³⁺, Ge⁴⁺, In³⁺, As³⁺, Sn²⁺,         Sn⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Cu¹⁺,         Cu²⁺, Zn²⁺, Cd²⁺ and mixtures thereof.     -   (11) Process according to item 9 or 10, wherein the at least one         radiometal is selected from the group consisting of ⁴⁴Sc, ⁴⁶Sc,         ⁵⁵Co, ^(99m)Tc, ²⁰³Pb, ⁶⁶Ga, ⁶⁷Ga, ⁶⁸Ga, ⁷²As, ¹¹¹In, ^(113m)In,         ^(114m)In, ⁹⁷Ru, ⁶²Cu, ⁶⁴Cu, ⁵²Fe, ^(52m)Mn, ⁵¹Cr, ¹⁸⁶Re, ¹⁸⁸Re,         ⁷⁷As, ⁹⁰Y, ⁶⁷Cu, ¹⁶⁹Er, ^(117m)Sn, ¹²¹Sn, ¹²⁷Te, ¹⁴²Pr, ¹⁴³Pr,         ¹⁹⁸Au, ¹⁹⁹Au, ⁴⁹Tb, ¹⁶¹Tb, ¹⁰⁹Pd, ¹⁶⁵Dy, ⁴⁹Pm, ¹⁵¹Pm, ¹⁵³Sm,         ¹⁵⁷Gd, ⁶⁶Ho, ¹⁷²Tm, ¹⁶⁹Yb, ¹⁷⁵Yb, ¹⁷⁷Lu, ¹⁰⁵Rh, ¹¹¹Ag and         mixtures thereof, preferably selected from the group consisting         of ⁴⁴Sc, ^(99m)Tc, ⁶⁷Ga, ⁶⁸Ga, ¹¹¹In, ⁶⁴Cu, ¹⁸⁸Re, ⁹⁰Y, ¹⁷⁷Lu         and mixtures thereof.     -   (12) Method of using chelate ligands according to any of items 1         to 4 or chelates according to any of items 6 to 8 in molecular         imaging.     -   (13) The method according to item 12, wherein molecular imaging         is multimodal molecular imaging. 

The invention claimed is:
 1. A chelate ligand having one of the following formulae


2. A chelate comprising at least one chelate ligand as defined in claim 1 and at least one metal or radiometal.
 3. The chelate according to claim 2, wherein the at least one metal or radiometal is selected from the group consisting of La³⁺, Ce³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu²⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, Lu³⁺, Sc³⁺, Y^(3′), Al³⁺, Ga³⁺, Ge⁴⁺, In³⁺, As³⁺, Sn²⁺, Sn⁴⁺, Sc³⁺, Ti⁴⁺, V⁵⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Cu¹⁺, Cu²⁺, Zn²⁺, Cd²⁺ and mixtures thereof.
 4. The chelate according to claim 2 or 3, wherein the at least one radiometal is selected from the group consisting of ⁴⁴Sc, ⁴⁶Sc, ⁵⁵Co, ^(99m)Tc, ²⁰³Pb, ⁶⁶Ga, ⁶⁷Ga, ⁶⁸Ga, ⁷²As, ¹¹¹In, ^(113m)In, ^(114m)In, ⁹⁷Ru, ⁶²Cu, ⁶⁴Cu, ⁵²Fe, ^(52m)Mn, ⁵¹Cr, ¹⁸⁶Re, ¹⁸⁸Re, ⁷⁷As, ⁹⁰Y, ⁶⁷Cu, ¹⁶⁹Er, ^(117m)Sn, ¹²¹Sn, ¹²⁷Te, ¹⁴²Pr, ¹⁴³Pr, ¹⁹⁸Au, ¹⁹⁹Au, ¹⁴⁹Tb, ¹⁶¹Tb, ¹⁰⁹Pd, ¹⁶⁵Dy, ¹⁴⁹Pm, ⁵¹Pm, ¹⁵³Sm, ¹⁵⁷Gd, ¹⁶⁶Ho, ¹⁷²Tm, ¹⁶⁹Yb, ¹⁷⁵Yb, ¹⁷⁷Lu, ¹⁰⁵Rh, ¹¹¹Ag and mixtures thereof, preferably selected from the group consisting of ⁴⁴Sc, ^(99m)Tc, ⁶⁷Ga, ⁶⁸Ga, ¹¹¹In, ⁶⁴Cu, ¹⁸⁸Re, ⁹⁰Y, ¹⁷⁷Lu and mixtures thereof.
 5. A method of molecular imaging comprising administering a chelate ligand or chelate, according to either claim 2 or claim 3, to a patient and performing an imaging step.
 6. The method according to claim 5, wherein said molecular imaging is multimodal molecular imaging of the distribution of said chelate ligand or chelate in said patient. 